The invention contemplates the improvement in systems for the heat-treatment of a continuous travelling length of a metal sheet or band which is heated by electrical currents passing therethrough in the course of its travel through a plurality of stages between conveyor or guide rolls or pulleys to which are applied electrical potentials.
The invention seeks to improve upon such systems as are disclosed in the patents of the prior art, and particularly such as are disclosed in the patent to Cook, U.S. Pat. No. 2,457,870 and others. While this patent discloses the resistive heating of electrically conductive wire in successive stages of shorter lengths to compensate for the increased resistivity of the wire in its travel from the inlet to the outlet of the system, with the use of alternating current energy, serious problems arise when such an expedient is adapted to the resistive heating of lengths of conductive material of wide area or those having a substantial width to thickness ratio, when such are enclosed within metallic chambers for protective gases used in the heat treatment of the material.
The instant invention seeks to overcome these problems by use of direct-current energy which prevents the induction of any currents in the walls of the sheet iron ductwork defining the chambers which surround the travelling band, thereby increasing the efficiency of the installation as well as minimizing the initial cost and the maintenance costs thereof. The direct current is applied to at least three electrified pulleys, each successive pulley having an opposite polarity.
The use of direct-current makes possible the placement of the sheet iron ductwork close to the travelling band, so that the radiant heat emanating from the latter is confined within a relatively small space and the quantities of gas which react with the travelling sheet and/or the coatings formed thereon may be reduced in quantity, as a consequence. Thus, the chambers for housing the travelling metal, which require no source of extraneous heat, are characterized by minimal thermal inertia and are capable of rapid shut-downs and re-starting operations, without substantial loss of time, energy and gases.
It is the object of the present invention to provide a highly compact and economical installation for the heat treatment of continuous lengths of metal bands or sheets for the purpose of imparting accurately controlled degrees of heat thereto for the purpose of modifying the physical and/or metallurgical properties of the metal, which installation may be complemented by additional apparatus for tempering, annealing or chemically treating the metal for further processing such as quenching, pickling or coating procedures.
It is a further object of the invention to provide an apparatus for the heat treatment of continuous lengths of metal bands or sheets, which occupies a minimum amount of floor area, which may be built up of low cost modular structural units, and which may be maintained in service for maximum periods of time without costly shut-downs when interruptions or break-downs occur.
It is a further object of the invention to provide an installation which is of particular utility in the heat treatment of continuous lengths of ferrous metal in the form of sheets, bands or strips, which are heat treated preparatory to the coating thereof with another metal such as aluminum, zinc, tin or the like, which procedure requires the effective cleaning of the surface of the metal to remove the oxides therefrom. This requires the passage of the continuous length of metal through chambers containing a protective gaseous atmosphere which is non-oxidizing or reducing in chemical behavior, which treats the travelling length of metal in the course of its advance towards a molten metal coating bath. The protective gas is introduced into the chambers for travel in countercurrent relationship to the direction of the travelling length of metal, to increase the efficiency of the system as the metal is first heated accurately to the desired temperature, followed by the cooling thereof and the hot dipping of the metal for the application of the coating thereto, in the course of its passage from the inlet to the outlet of the apparatus.
The invention contemplates the economical heat treatment of continuous lengths of ferrous metal preparatory to the passage thereof through coating baths of molten metal which are treated for the purpose of clearing the metal of objectionable oxide layers, with or without the annealing of the metal. Alternatively, the heat treatment of the continuous lengths of ferrous metal may be executed preparatory to the passage of the critically heated metal through quenching baths, if tempering characteristics are sought to be imparted to the metal, or other liquid baths such as pickling solutions and the like.